1. Field of the Invention
The present invention generally relates to wireless systems, and more specifically to techniques for providing diversity in a wireless system.
2. Background
Diversity is the use of multiple versions of a signal in a system. Diversity typically improves the performance of the system because another version of the signal is available if a first version encounters a problem. Multiple versions of the signal can be provided by hardware and/or software, for example. Digital signal processing (DSP) techniques are often employed to provide the multiple versions. Wireless communication systems in particular can employ and take advantage of various types of diversity. Some examples are temporal diversity, whereby the system utilizes different copies of the signal in time, and frequency diversity, whereby the system utilizes different copies of the signal in frequency. For a wireless communication system that employs multiple antennas, spatial diversity can also be utilized by the system, whereby different copies of the signal are present on each antenna.
Conventional wireless systems, such as those based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard (hereinafter referred to as “legacy wireless systems”), either are not designed to have spatial diversity or can only use spatial diversity in a limited manner. A legacy wireless system, for example, includes a single transmitter and a single receiver. The transmitter encodes data before transmitting the data, and the receiver decodes the encoded data for further processing. If the receiver has only a single antenna then it cannot benefit from spatial diversity. If it has multiple receive antennas, it can employ some suboptimal algorithm to choose the antenna having the highest receive power, e.g., in order to enhance the received signal strength. Similarly, if the transmitter has only one antenna it cannot use spatial diversity. If it has multiple transmit antennas it can employ some suboptimal algorithm that chooses one of the antennas based, e.g., on the result of previous receptions on that antenna
Some modern wireless systems include multiple transmitters to improve the transmission rate, the range, and/or the reliability of the wireless system. For instance, a proposed IEEE wireless local area network (WLAN) standard, IEEE 802.11n, allows a transmission rate of up to 130 Mbps in 20 MHz bandwidth by utilizing two transmitters. The proposed standard at least doubles the transmission rates achievable using other WLAN standards. For example, IEEE 802.11a and IEEE 802.11g each support a transmission rate of up to 54 Mbps.
Example wireless systems having multiple transmitters include multiple input, single output (MISO) systems and multiple input, multiple output (MIMO) systems. In a MISO system, multiple transmitters transmit data to a single receiver. In a MIMO system, multiple transmitters transmit data to multiple receivers.
In a conventional MISO or MIMO system, different transmitters transmit different data. If two or more transmitters in a conventional MISO or MIMO system transmit the same data, then the energy transmitted by each transmitter cancels the energy transmitted by the other transmitter(s) at locations that are based on the distance between the respective transmitters.
What is needed is a method, system, and/or computer program product that addresses one or more of the aforementioned shortcomings of conventional wireless systems having multiple transmitters.